This invention relates to meters for measuring the ventilatory capacity of a subject, particularly, but not necessarily exclusively the exhalation capacity.
Meters for obtaining a measure of the peak flow rate of exhalation are known (GB-A-1463814) in which the subject blows into one end of a tubular body to displace a piston, against the force of a spring, along the body. The piston is guided slidably for this displacement, out of contact with the body inner wall, on a support rod extending along the axis of the tube. The tube has an open slot running in the direction of piston displacement which provides an exit opening for the air being blown in. A pointer located behind the piston has a light frictional engagement with the slot and is displaced along a scale by the piston as it moves forward against the spring force. The piston is drawn back by the spring when the intensity of exhalation falls, but when it moves back, the pointer remains at the position of maximum displacement of the piston, so giving an indication of the maximum flow rate obtained in the exhalation.
Such meters have been developed as reliable and robust instruments. However, the presence of the central support rod for guiding the piston complicates the assembly process.
In one of its aspects, the present invention provides a meter for measuring the ventilatory capacity of a subject, the meter comprising a chamber, a piston within the chamber being displaceable axially against a resilient bias by blowing air into the chamber, an exit slot located in a side of the chamber for the escape of the air from the chamber being increasingly opened by the displacement of the piston against the bias, there being a region of sliding contact between the piston and the chamber inner wall at any one position of the piston and the sliding contact region having bounds separated axially by a distance not substantially less than 25% of the transverse dimension of the piston, preferably at least 30%, the sliding contact guiding the movement of the piston and preventing tilting of the piston, and there being means for recording a maximum displacement of the piston. Preferably the axial extent of the periphery is 50% or more of the transverse dimension of the piston, i.e. for a cylindrical piston, its diameter.
By giving the contact region a sufficient axial extent it is possible to avoid the need to mount the piston on a support rod, i.e. the piston is a free piston. The assembly of the meter is simplified and its construction costs reduced.
The piston periphery may take the form of a cylindrical wall of complementary form to and in sliding contact with the chamber inner wall. However, while such a configuration can prevent tilting of the piston, it can affect adversely the accuracy and repeatability of the meter, particularly at low levels of air flow. One reason for this may be that in use, condensation or other matter may be deposited from the exhalations and can accumulate on the periphery of the piston.
These effects could be lessened by increasing the nominal clearance between the piston and the chamber wall. However, the piston is then more likely to tilt and possibly jam within the chamber. The problem can be solved by making the contact between piston and the inner wall peripherally discontinuous e.g. by the provision of ribs on the piston or on the wall: thus the contact area is lessened but resistance to tilting can be maintained.
According to another aspect of the invention a meter for measuring the ventilatory capacity of a subject comprises a chamber, a piston within the chamber displaceable against a resilient bias by blowing air into the chamber, an exit slot located in a side of the chamber for the escape of the air from the chamber being increasingly opened by the displacement of the piston against the bias, portions supporting the piston on the chamber wall against tilting and arranged to engage between the piston and the chamber wall over only a part of the axial and/or circumferential extent of the periphery of the piston; and means for recording a maximum displacement of the piston in the chamber. Thus the piston may be a free piston, i.e. be one devoid of support except from the chamber wall.
Thus, the contact region can comprise a plain, e.g. cylindrical, wall or a plurality of axially spaced peripheral rims. At least one of the peripheral rims may have a complementary profile to the internal cross-sectional form of the chamber. Alternatively, the region can comprise a plurality of circumferentially spaced axially extending ribs. In another form, an axially restricted disc of a piston is complemented by a plurality of ribs forming axial extensions increasing the axial contact length between the piston and the chamber wall.
In these and similar forms of piston, as well as in the case in which the piston has a cylindrical wall, the co-acting wall of the chamber may be formed so that there is contact only at angularly spaced regions. For example, a circular piston may be mounted in a chamber which has a non-circular cross-section or a non-circular piston mounted in a circular cross-section chamber. Alternatively, the chamber may have inwardly extending projections for slidably guiding the piston.
The resilient bias acting on the piston should be applied in a manner that does not produce any significant non-axial force that might cause the piston to tend to tilt. If a coil spring is employed as the biasing device, it is therefore preferably attached centrally to the piston.